1. Field of the Invention
The present invention relates to an electric power steering control apparatus for generating, by means of a motor, an assist torque for supplementing a steering torque exerted by a driver.
2. Description of the Related Art
A conventional steering wheel control structure for a motor-steered vehicle is equipped with an electric power steering apparatus for generating, by means of an electric motor, an assist steering torque for turning steering wheels in accordance with an assist steering torque signal (i.e., steering assist torque) output from steering assist torque determining/inputting means (i.e., steering assist torque calculating means). This steering wheel control structure has a left-and-right wheel control means and an anti-torque steerage assist torque determining means (i.e., external device).
The left-and-right wheel control means controls a driving force or a braking force applied to each of left wheels and each of right wheels independently. When there is generated a difference between a control value used by the left-and-right wheel control means to control the driving force or the braking force applied to each of the left wheels and a control value used by the left-and-right wheel control means to control the driving force or the braking force applied to each of the right wheels, the anti-torque steerage assist torque determining means outputs an anti-torque steerage assist torque signal (i.e., addition command torque) for turning the steering wheels in such a direction as to counterbalance torque steerage generated due to the difference between the control values.
The motor is controlled by a signal obtained by adding the assist steering torque signal and the anti-torque steerage assist torque signal together (e.g., see JP 11-129927 A).
A conventional electric steering control apparatus is equipped with a steering torque control means (i.e., steering assist torque calculating means) for controlling a steering torque applied to steering wheels of a vehicle in accordance with a steering operation. This electric power steering control apparatus has a braking force estimating means, a left-and-right braking force difference estimating means, and an assist steering torque applying means.
The braking force estimating means estimates braking forces applied to the respective wheels of the vehicle. The left-and-right braking force difference estimating means estimates a difference between the braking forces applied to each of the left wheels and each of the right wheels, based on the braking forces estimated by the braking force estimating means. The assist steering torque applying means applies an assist steering torque (i.e., addition command torque) to the steering torque control means in accordance with the difference between the braking forces applied to each of the left wheels and each of the right wheels, which have been estimated by the left-and-right braking force difference estimating means (e.g., see JP 2003-291838 A).
A conventional lane traveling support apparatus for a vehicle is equipped with a steering control means capable of operating in accordance with a steering wheel operation performed by a driver and controlling a steering state in accordance with a state in which the vehicle travels on a road surface. This lane traveling support apparatus has a driving lane detecting means, a vehicle state estimating means, a target state quantity setting means, and a disturbance estimating means.
The driving lane detecting means detects a driving lane from images of a road surface continuously captured by an image pickup means. The vehicle state estimating means, which has a vehicle movement model and a road model, estimates a state quantity including a lateral position of the vehicle within the driving lane in accordance with a result detected by the driving lane detecting means, a steering state of the vehicle, and a traveling state of the vehicle, based on the vehicle movement model and the road model. The target state quantity setting means sets a target state quantity for the vehicle based on the steering state and the traveling state of the vehicle. The disturbance estimating means estimates a disturbance amount for the vehicle based on the state quantity estimated by the vehicle state estimating means.
An addition steering torque command value (i.e., addition command torque) is applied to the steering control means based on a difference between the target state quantity set by the target state quantity setting means and the state quantity estimated by the vehicle state estimating means, and on the disturbance amount estimated by the disturbance estimating means. Thus, the vehicle is supported in traveling within the driving lane (e.g., see JP 2005-125853 A).
In the conventional device disclosed in JP 11-129927 A, the value obtained by adding together the steering assist torque for supplementing the steering force exerted by the driver by turning the steering wheels and the addition command torque input from the external device to turn the steering wheels in such a direction as to counterbalance the torque steerage is used as the target assist torque of the motor.
The steering assist torque is calculated by multiplying the steering torque by a gain corresponding to a vehicle speed in a steering assist torque calculating portion. The vehicle speed and the steering torque are input, respectively, from a vehicle speed detector and a steering torque detector, which are connected to the steering assist torque calculating portion.
The operation of the conventional apparatus disclosed in JP 11-129927 A will be described hereinafter with reference to an explanatory diagram of FIG. 6.
In a case where the addition command torque has been changed from “0” to “1” during straight-ahead traveling of the vehicle and the driver has continued to travel straight ahead while holding the steering wheel, the steering torque, the steering assist torque, the addition command torque, and the target assist torque change as shown in FIG. 6 as time passes.
The addition command torque is set to “0”, changed from “0” to “1”, and set to “1” when an elapsed time is between 0 [sec] and 1 [sec], equal to 1 [sec], and longer than 1 [sec], respectively.
Referring to FIG. 6, the values of the respective torques corresponding to the elapsed time between 0 [sec] and 1 [sec] will be described first.
Since the vehicle is traveling straight ahead, the steering torque is “0”. The steering assist torque, which is calculated by multiplying the steering torque by the gain corresponding to the vehicle speed, is therefore “0”.
The addition command torque is set to “0” when the elapsed time is between 0 [sec] and 1 [sec]. The target assist torque, which is calculated by adding the steering assist torque and the addition command torque together, is therefore “0”.
Then, when the addition command torque is changed from “0” to “1” as the elapsed time reaches 1 [sec], the target assist torque, which is calculated by adding the steering assist torque and the addition command torque together, is therefore “1”.
At this moment, the motor is driven with the target assist torque held at “1”, and a motor torque “1” is applied to a steering system.
Next, the values of the respective torques immediately after the change in the addition command torque from “0” to “1”, which corresponds to the elapsed time equal to 1 [sec], will be described.
The driver continues to travel straight ahead while holding the steering wheel, so the steering torque is equal to a reaction force of the motor torque “1”, namely, “−1”. The steering assist torque, which is calculated by multiplying the steering torque by the gain corresponding to the vehicle speed, is therefore equal to a value other than “0”.
The addition command torque, which is set to “1” when the elapsed time is longer than 1 [sec], is therefore “1”. The target assist torque, which is calculated by adding the steering assist torque and the addition command torque together, is therefore equal to a value other than “1”. The motor is driven with the target assist torque equal to the value other than “1”, and a motor torque corresponding to this target assist torque acts on the steering system.
Subsequently, the values of the respective torques corresponding to the elapsed time longer than 1 [sec] will be described.
The driver continues to travel straight ahead while holding the steering wheel, and the steering torque, which is equal to a reaction force of the motor torque, is therefore equal to a value other than “−1”.
The same operations are thereafter repeated, so the steering torque, the steering assist torque, and the addition command torque converge to values satisfying a relationship: (steering torque+steering assist torque)=−(addition command torque).
As described above, in the case where the addition command torque has been changed from “0” to “1” during, for example, straight-ahead traveling of the vehicle and the driver has continued to travel straight ahead while holding the steering wheel, the steering torque detector detects the reaction force of the motor torque generated due to the change in the addition command torque, so the steering assist torque is generated.
Thus, the ultimate target assist torque (i.e., target assist torque corresponding to the elapsed time longer than 1 [sec] in FIG. 6) does not become equal to “1”.
The addition command torque is originally intended to add a predetermined torque to the steering assist torque. However, as described above, when the steering assist torque changes due to the addition of the addition command torque, a change amount of the addition command torque does not coincide with a change amount of the motor torque. Consequently, the object of adding the predetermined torque to the steering assist torque cannot be achieved.
In setting the addition command torque by using the external device, therefore, it is necessary to perform two operations of adjusting an applied torque amount, namely, an operation of adjusting a required applied torque amount and an operation of adjusting an actual applied torque amount (i.e., addition command torque) required for actual generation of the applied torque amount.
The same holds true for JP 2003-291838 A and JP 2005-125853 A.
In the conventional steering wheel control structure for the motor-steered vehicle, the change amount of the addition command torque does not coincide with the change amount of the motor torque, so the two operations of adjusting the applied torque amount need to be performed. As a result, there is a problem in that a considerable length of time is required.